This invention relates to abrasive wheels generally and, more particularly, to abrasive wheel assemblies having an abrasive wheel and a mounting hub for use as grinding or cut-off wheels on portable and stationary machines.
A well-known type of abrasive wheel assembly is commonly known as a depressed center type 27 or type 28 wheel. Typically, such wheel assemblies are used on portable hand-held machines for grinding or cut-off work. In almost all cases, a disposable die cast zinc hub permanently fixed to the abrasive wheel is used to mount the abrasive wheel on a grinding or cut-off machine. The offset of the depressed center of the wheel permits mounting of the hub on the abrasive wheel without protrusion of the mounting means beyond the face of the wheel. The abrasive wheel may be made of woven fiberglas combined with a suitable abrasive.
In such wheel assemblies, there are two mounting requirements for adequately securing the hub to the wheel. One requirement is to provide clamping force; this is accomplished by riveting or spinning a sleeve portion of the hub over a central aperture of the wheel. The second requirement is to secure the hub rotationally; this is normally done by using an epoxy resin between a flange portion of the hub and the raised portion of the wheel defining the offset. Because of the high rotational speeds--8500 rpm or more, depending on wheel diameter--the wheel may fly apart or "explode" if either requirement is not met.
The common reliance on an epoxy mounting of the hub has several weaknesses. The surface preparation, intermixing of the epoxy resin, and amount of epoxy resin are critical to a satisfactory bond. Further, the alignment and balance of the abrasive wheel must be maintained to prevent wheel wobble. Because the epoxy resin is not visible, it is very difficult to determine from an inspection of the wheel assembly whether the hub is secure rotationally. In order to test the epoxy bond for maximum strength, the assembly must be destroyed. Destructive testing of a representative sample is used to minimize the number of defective wheel assemblies which are sold, but not all wheel assemblies with unsatisfactory bonds are located by such testing.
Applicant is aware of several reports of mounting failure of such wheel assemblies and is aware of numerous recalls of wheel assemblies by manufacturers in which particular batches were defective. Applicant tested a representative sample of 75 wheel assemblies manufactured by four of the major suppliers: Bay State of Massachusetts, Brilliant Abrasive of Kansas City, Ace Abrasive of Troy, Ohio, and Carborundum of Niagara Falls, N.Y. Commercially available 9-inch diameter depressed center wheel assemblies were subjected to severe edge and side loading considered to be in excess of that of normal use. In 14 of the 75 samples, the epoxy bond failed in various degrees before the useful life of the abrasive wheel was reached.
Due to the extremely hazardous condition presented by any defect in the epoxy bond between the hub and the wheel, there is a need for an improved mounting of the hub less dependent on an epoxy bond. One attempt to meet this need is a three-pin glueless hub disclosed in Foundry Equipment News magazine, June 1980 at page 19. The proposed pins are longitudinal, dog-like protrusions along the circumference of the hub placed so as to penetrate three corresponding holes in the abrasive material of the wheel. However, the proposed mounting may result in wearing or weakening of the wheel at the three holes penetrated by the pins and requires costly additional manufacturing operations.
It is therefore an object of the present invention to provide an inexpensive abrasive wheel assembly having greater safety than that of a conventional wheel assembly relying on an epoxy bond to secure the hub rotationally. It is a further object of the present invention to provide an abrasive wheel assembly having an improved mounting of the hub to the wheel permitting greater loading and increased useful life of the wheel assembly. It is still a further object of the present invention to provide a hub for an abrasive wheel assembly having improved control over eccentricity and wobble of an abrasive wheel to which it is secured.